See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/313126430 Fracture network, fluid pathways and paleostress at the Tolhuaca geothermal field Article in Journal of Structural Geology · January 2017 DOI: 10.1016/j.jsg.2017.01.009 CITATIONS READS 12 810 6 authors, including: Pamela Perez-Flores Eugenio E Veloso Pontificia Universidad Católica de Chile Pontificia Universidad Católica de Chile 18 PUBLICATIONS 219 CITATIONS 57 PUBLICATIONS 613 CITATIONS SEE PROFILE SEE PROFILE Jose Cembrano Pablo Sánchez Pontificia Universidad Católica de Chile Universidad Austral de Chile 149 PUBLICATIONS 3,682 CITATIONS 19 PUBLICATIONS 299 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: IODP1256D View project Fulbright research View project All content following this page was uploaded by Jose Cembrano on 05 February 2018. 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Journal of Structural Geology 96 (2017) 134e148 Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Fracture network, fluid pathways and paleostress at the Tolhuaca geothermal field * Pamela Perez-Flores a, b, , Eugenio Veloso a, b, Jose Cembrano a, b, Pablo Sanchez-Alfaro b, c, Martín Lizama b, c, Gloria Arancibia a, b a Departamento de Ingeniería Estructural y Geotecnica, Pontificia Universidad Catolica de Chile, Avenida Vicuna~ Mackenna 4860, Macul, Santiago, Chile b Andean Geothermal Center of Excellence (CEGA, FONDAP-CONICYT), Santiago, Chile c Departamento de Geología, Universidad de Chile, Santiago, Chile article info abstract Article history: In this study, we examine the fracture network of the Tolhuaca geothermal system located in the Received 4 October 2016 Southern Andean volcanic zone that may have acted as a pathway for migration and ascent of deep- Received in revised form seated fluids under the far/local stress field conditions of the area. We collected the orientation, slip- 23 January 2017 data and mineralogical content of faults and veins recovered on a ca. 1000 m deep borehole (Tol-1) Accepted 25 January 2017 located in the NW-flank of the Tolhuaca volcano. Tol-1 is a non-oriented, vertical borehole that recovered Available online 30 January 2017 relatively young (<1 Ma) basaltic/andesitic volcanic rocks with subordinate pyroclastic/volcanoclastic interbedded units of Pleistocene age. Here, we examined and measured the inclination, geometry, Keywords: Tolhuaca geothermal field texture, mineralogy, and relative sense of displacement of veins and faults. To determine the actual Paleomagnetic core reorientation azimuthal orientation of fault and veins we reoriented 66 segments (89 standard mini-cores) of Tol-1 Stress field using stable Characteristic remanent magnetization component (ChRM) obtained by thermal demag- Liquine-Ofqui~ fault system netization methodology. Paleo-declination of ChRM vectors was used to re-orient the borehole pieces, as well as fault and veins, to a common anchor orientation value consistent with the Geocentric Axial Dipole approximation (GAD). Inversion of RM-corrected fault-slip data reveals a local tensional stress field with a vertically oriented s1 axis (083/74) and a subhorizontal, NS-trending s3 axis (184/03). Within the topmost 400 m of the borehole, faults and veins are randomly oriented, whereas below 400 m depth, faults and veins show preferential NE-to EW-strikes and steep (>50) dips. The EW-striking veins are compatible with the calculated local stress field whereas NE-striking veins are compatible with the regional stress field, the morphological elongation of volcanic centers, alignments of flank vents and dikes orientation. Our results demonstrate that the paleomagnetic methodology proved to be reliable and it is useful to re-orient vertical boreholes such as Tol-1. Furthermore, our data show that the bulk transpressional regional stress field has local variations to a tensional stress field within the NE-striking fault zone belonging to the Liquine-Ofqui~ Fault System, favoring the activation of both NW- and NE- striking pre-existent discontinuities, especially the latter which are favorably oriented to open under the prevailing stress field. The vertical s1 and NS-trending subhorizontal s3 calculated in the TGS pro- mote the activation of EW-striking extensional veins and both NE and NW-striking hybrid faults, constituting a complex fluid pathway geometry of at least one kilometer depth. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Tectonic activity in areas of intense heat flux, controls the dy- * Corresponding author. Departamento de Ingeniería Estructural y Geotecnica, namics of deformation, fluid-flow ascent and heat transfer (e.g. fi ~ Ponti cia Universidad Catolica de Chile, Avenida Vicuna Mackenna 4860, Macul, Barnhoorn et al., 2010; Nakamura, 1977; Sibson, 1996). These pro- Santiago, Chile. cesses lead to the generation of structural elements (fault, veins, E-mail addresses: [email protected] (P. Perez-Flores), [email protected] (E. Veloso), [email protected] (J. Cembrano), [email protected] and joints), which are arranged as a complex fracture network. (P. Sanchez-Alfaro), [email protected] (M. Lizama), [email protected] These structural elements are important pathways for the (G. Arancibia). http://dx.doi.org/10.1016/j.jsg.2017.01.009 0191-8141/© 2017 Elsevier Ltd. All rights reserved. P. Perez-Flores et al. / Journal of Structural Geology 96 (2017) 134e148 135 migration, ascent and/or emplacement of deep-seated fluids in transpressional regional tectonic regime, characterized by N65E- hydrothermal systems because primary permeability and porosity trending s1 favors dextral/oblique displacement along NNE- is continuously sealed by hydrothermal mineralization (e.g. Bons striking faults, NE-ENE-striking extensional fractures and hybrid et al., 2012; Rowland and Sibson, 2004; Zhang et al., 2008). faults, and sinistral-reverse NW-striking faults (Lavenu and Examining the nature, spatial distribution, and geometry of such Cembrano, 1999; Perez-Flores et al., 2016). Strain partitioning at fracture network can help to unravel the fundamental geological these different tectonic domains control the chemistry of the vol- processes operating on both ancient and active geothermal systems canic products, the geometric and spatial arrangement of volcanic (e.g. Brogi, 2008; Curewitz and Karson, 1997; Rowland et al., 2012). centers, as well as the hydrothermal activity in the intra-arc zone Surface structural mapping of fault-fracture networks in active (Lara et al., 2006; Perez-Flores et al., 2016; Radic, 2010; Sanchez geothermal systems helps understanding the overall geometry and et al., 2013; Sepúlveda et al., 2005; Sielfeld et al., 2016; Tardani preferential orientation of fluid pathwayse and eventually the et al., 2016). nature of heat transfer e yet these are only proxies of the actual Although these major faults have been widely recognized in the conditions operating at depth (Nukman and Moeck, 2013). Sub- field at various scales, there is no published information on the surface information can be collected using geophysical methods or subsurface distribution and orientation of structural elements in by means of coring and drilling. Structural elements contained on and around the TGF (Lohmar et al., 2012; Melosh et al., 2012, 2010). recovered whole rock drilling pieces can then be spatially oriented In this contribution, we report new structural data collected from a with respect to the core axis, providing valuable inclination-only vertical, non-oriented, ca. 1080 m deep, borehole drilled in the NW data about the general and downhole distribution of geological flank of the Tolhuaca volcano (Tol-1). GeoGlobal Energy LLC kindly elements (e.g. Dobson et al., 2003; Ganerød et al., 2008; Moncada provided the Tol-1 drill core sampled during an exploration et al., 2012). However, random rotations of the rock pieces about campaign held in 2012. We analyze standard mini-cores from the the axis of the core prevent direct measurements of azimuthal data. Tol-1 core using paleomagnetic techniques allowing the isolation of Then, it becomes necessary to re-orient recovered rock pieces into characteristic remanent magnetic vectors, thus providing a tool for the geographic coordinate system, to further re-orient measured the re-orientation of the drilled core. The orientation and charac- structural elements. Re-orientation techniques are varied (e.g. teristics of the local strain and stress fields of the TGF were then MacLeod et al., 1994; Ureel et al., 2013; Virgil et al., 2015) and estimated with re-oriented fault-slip data. include the following: (1) drawing of a reference line as drill-bits are recovered; (2) plaster pieces on bottom to mark the spatial 2. Geological and tectonic framework position of a certain piece or section of the core; (3) geophysical imaging of the drilled hole, and (4) paleomagnetic re-orientation The Tolhuaca volcano is located at the intersection of the (e.g. Didenko, 1996; Virgil et al., 2015; Zhang et al., 2007). Paleo- northern termination of the LOFS and a NW-striking fault of the ATF magnetic core re-orientation has been successfully used in hydro- (Fig. 1a). Major structural elements present around of the volcanic carbon exploration projects defining fracture orientations edifice are spatially related to one of these faults, which could have confirmed by borehole image logging, mostly in sedimentary